The present invention relates to an automatic speed shifting device which employs a torque feedback device to change the position of a shift gear so as to engage corresponding shifting member in the transmission device. The speed reduction device automatically shifts the transmission device when the load torque increases or reduces.
A conventional power transmission device, especially for electric spinning tools, such as electric drills and electric screwdrivers, includes a multiple-stage power transmission device and a speed reduction device is used to provide multiple speeds or torque. Generally, the speed reduction device is composed of a planetary gear system and clutch or driving members in the speed reduction device are manually controlled such that some parts are fixed or moved in the planetary gear system and the purposes of speed reduction of output or input can be achieved. An operator has to judge the situation of the tool and then decide to operate the manual device to activate proper speed reduction device to obtain desired torque or revolutions of speed. It is difficult to make the efficiency of the driving motor optimized by the manual operation. Therefore, a feedback device for the load torque is needed so as to shift proper steps of the speed reduction device.
In accordance with an aspect of the present invention, there is provided an automatic speed shifting device that comprises a frame having a transmission device and a torque feedback device received therein. The frame has a plurality of triangle shaped slots defined through a wall thereof. A radial groove is defined through the wall of the frame. A plurality of protrusions extending from an inside of the frame and ridges are defined on an outer surface of the frame. The transmission device has a shifting gear that has inner teeth that are engaged with first planet gears and second planet gears. The shifting gear has an annular groove with which a plurality of pins on a clamp are engaged so as to retain the shifting gear in a first stage speed position and to engage the shifting gear with the two planet gears, or retain the shifting gear in a second stage speed position and only engage the shifting gear with the second planet gears. The torque feedback device has a pushing wheel, a C-shaped clamp and a compression spring. The pushing wheel has a lever extending from an outer surface thereof and the clamp has a plurality of pin extending through the slots of the frame and engaged with the annular groove of the shifting gear. An elongate hole is defined through the clamp and located corresponding to the radial groove of the frame and the lever of the pushing wheel. The compression spring is mounted to the frame and retained between rides and the ridges on the frame. When the pushing wheel is rotated, the lever is moved in the radial groove of the frame and drives the clamp via the elongate hole in the clamp. The clamp compresses the compression spring by a movement along an inclined surface of the radial groove of the frame so as to generate resistant force.
When the torque applied on the pushing wheel from the front speed reduction gear cannot overcome the resistant force from the torsion spring and the compression spring, the shifting gear is in its first stage of speed status and is engaged with the first planet gears and the second planet gears. When the torque applied onto the pushing wheel overcomes the resistant force from the torsion spring and the compression spring, the pushing wheel rotates and drives the clamp to rotate the shifting gear, and the shifting gear is not rotated due to the engagement of the notches and the protrusions.
The present invention will become more obvious from the following description when taken in connection with the accompanying drawings which show, for purposes of illustration only, a preferred embodiment in accordance with the present invention.